1. Field of the Invention
The present invention relates to the purification of solutions or suspension of impure polyorganophosphazenes.
2. Description of the Prior Art
The polyorganophosphazenes are known polymers comprising a plurality of recurring structural units of the formula: ##STR1## in which the symbols R, which may be identical or different in the same recurring unit or in different recurring units, are advantageously alkyl, aryl, alkoxy, fluoroalkoxy, aryloxy, alkyl- or arylsulfide, or alkyl- or arylamino radicals.
The polymers of formula (I) will be referred to hereinafter as polyorganophosphazenes or, more simply, as polyphosphazenes.
These polymers are prepared by substituting the chlorine atoms of polydichlorophosphazenes with nucleophilic reagents according to the following reaction: ##STR2## wherein X is a hydrogen atom or an alkali metal and R is as defined above.
This substitution reaction is carried out in solution and, in order to assure complete conversion, the operation is generally carried out in the presence of an agent that is nucleophilic relative to the chlorine atom to be substituted. The separation of these polymers in the pure state thus entails the elimination of the solvent, the excess reagents and the metallic salts of the nucleophilic agents after substitution.
For the elimination of metallic salts, and more particularly the elimination of NaCl, an extraction process comprising washing with water (JP 59/45,324) or using a mixture of isopropanol and water (U.S. Pat. No. 4,576,806) is known to this art. Such a process is obviously not applicable in the case of a water-soluble polyphosphazene.
To eliminate the solvent and the excess reagents, one such method entails precipitating the polymers by pouring a solution thereof into a precipitating liquid and repeating this operation until the desired purity is attained (H. R. Allcock et al, Inorg. Chem., 5 (10), 1709 (1966). This process requires the use of at least two types of solvents, which subsequently have to be separated, and which becomes increasingly expensive with increasing volumes to be treated, in order to provide complete purification. Furthermore, as the most common physical state of polyphosphazene is that of a gum, it is generally not possible to simply conduct the operation in a conventionally agitated reactor, as the recovery of the gum, which often is adhesive, is very difficult.
In published French Application No. 87/14,215, a method is described which includes the distillation of the solvent directly in a heated mixer, optionally in a vacuum, and washing the polyphosphazene by comminution in the presence of a nonsolvent for the polymer.
In the particular case of the polyaryloxyphosphazenes, a process for the elimination of excess phenols is described in U.S. Pat. No. 4,789,728. It entails washing the polymer in solution in cyclohexane, with a hydroalcohol solution of sodium hydroxide. This process frequently gives rise to emulsions and makes it necessary to separate the phenols and the alcohol prior to discharging the water.